The present disclosure relates to bidirectional switching devices and bidirectional switching circuits using such bidirectional switching devices.
There has been a demand for electronic equipment which saves more energy, and it has been desired to improve the power conversion efficiency of power converters, such as a power supply, an inverter, a matrix converter, etc., which consume a large amount of power. In particular, a matrix converter directly converting AC power into AC power having a different frequency and voltage can convert AC power without conduction through a diode rectifier, and therefore, it can be expected to improve the power conversion efficiency, compared to conventional inverters. The matrix converter includes a bidirectional switch conducting a current flowing in two directions, and having a breakdown voltage with respect to positive and negative voltages. A bidirectional switch currently generally used includes two insulated gate bipolar transistors (IGBTs) connected in antiparallel, and two diodes each of which is connected to the corresponding one of the IGBTs in series.
It is important for a semiconductor element performing a bidirectional switching to reduce a switching loss expressed by a product of a transient voltage and a transient current at a time of switching, and a conduction loss consumed by a resistance of the semiconductor element itself (referred to as an on-state resistance) in the on state. However, when a bidirectional switching circuit is formed by using a silicon (Si) device, it has been difficult to reduce the on-state resistance due to a Si material limit.
In order to reduce the conduction loss beyond the Si material limit, it has been contemplated to introduce a semiconductor element using a wide-gap semiconductor made of a nitride semiconductor (e.g., gallium nitride (GaN) etc.), silicon carbide (SiC), etc. The wide-gap semiconductor has a dielectric strength higher than that of Si by about an order of magnitude. In particular, charge occurs at a heterojunction interface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN) due to spontaneous polarization and piezoelectric polarization. As a result, even if the layers are undoped, a two-dimensional electron gas (2DEG) layer is formed which has a sheet carrier concentration of 1×1013 cm−2 or more and a mobility of as high as 1000 cm2 V/sec or more. Therefore, an AlGaN/GaN heterojunction electric field effect transistor (AlGaN/GaN-HFET) has been expected to serve as a power switching transistor which achieves a low on-state resistance and a high breakdown voltage.
However, as well as conventional bidirectional switching circuits, even if an AlGaN/GaN-HFET is used for a bidirectional switching circuit, it is necessary to provide two AlGaN/GaN-HFETs and two diodes, and compared to the Si device, significant reduction of the on-state resistance cannot be expected.
In order to achieve a bidirectional switch having a lower on-state resistance, for example, International Patent Publication No. WO 08/062,800 proposes a bidirectional switching device which serves as a semiconductor element having double gates and in which one element can constitute a bidirectional switch.